Several years ago, we noted during electrophysiological studies that a number of neurons in the lumbar spinal cords of rats could be antidromically activated from the hypothalamus using small amounts of current. These findings suggested the existence of a projection that carries somatosensory and nociceptive information from the spinal cord directly to the hypothalamus. We used several anatomical and physiological techniques to confirm the existence of this projection. Interestingly, our retrograde tracing studies indicated that in rats the total numbers of spinohypothalamic tract and spinothalamic tract neurons are similar. In thoracic and sacral levels of the cord, there are considerably more spinohypothalamic tract neurons than spinothalamic tract neurons. The spinohypothalamic tract is of interest because d may be an important source of input to areas of the hypothalamus that are thought to play important roles in autonomic, neuroendocrine and affective responses to somatosensory stimuli. In the proposed experiments we will continue our studies of this projection. In the first series of studies, we will determine whether a spinohypothalamic tract exists in monkeys. This is an issue that should be examined directly because several somatosensory systems, including the spinohypothalamic tract, vary considerably among species. In the second set of experiments, we will use a powerful new double labeling technique to determine the number and location of spinal cord neurons that project to both the thalamus and hypothalamus in rats. In the third set of studies, we will use electrophysiological techniques in rats to determine the location of ascending spinohypothalamic tract axons as they ascend through the medulla, pons, and midbrain . These studies will also indicate whether spinohypothalamic tract axons issue collateral projections to areas in the brainstem and midbrain that are involved in somatosensory processing. If so, electrophysiological evidence of prominent collateral projections from SHT axons will be evaluated with double labeling studies. In the fourth and final set of studies, we will use anatomical and physiological techniques to examine the large number of spinohypothalamic tract neurons in the thoracic spinal cord. In these studies, we will determine the areas of the hypothalamus (and telencephalon) that receive a direct projection from neurons in thoracic cord. In addition, we will physiologically examine the responses of thoracic spinohypothalamic tract neurons to a variety of quantitative mechanical and thermal stimuli. We will also determine whether thoracic SHT neurons are activated by suckling pups. The proposed studies should considerably expand our knowledge about spinal cord neurons that project to the hypothalamus.